1. Field of the Invention
This invention relates to an industrially advantageous process for producing a propylene copolymer with high quality. More particularly, it is concerned with an industrially advantageous process for producing a propylene copolymer excellent in film transparency, stiffness, blocking properties, heat-sealing properties, etc., by using a particular Ziegler-Natta catalyst with a particular composition.
2. Description of the Prior Art
Isotactic polypropylene produced using a stereo-regular catalyst is widely used for various moldings due to its excellent stiffness, strength, molding properties, appearance, and heat resistance. Polypropylene films are popularly used as various wrapping materials as a result of their highly valued transparency and firmness. However, these polypropylenes suffer from several disadvantages. One defect is their largely temperature-dependent impact strength. Their so-called freeze resistance is poor. That is, impact strength is sharply reduced between room temperature and 0.degree. C. Another defect is their high heat-sealing temperature. Using a biaxially oriented film, for example, the temperature required to obtain a sufficient heat-sealing strength is so high, it spoils the film appearance. As a result, heat-sealing such films is in most cases impossible.
In order to overcome these problems, random copolymers containing a small amount of .alpha.-olefin such as ethylene or butene-1 (e.g., propylene-ethylene copolymer, propylene-butene-1 copolymer, propylene-ethylene-butene-1 copolymer, etc.) have been used alone or as blends with other resins or rubbers, as heat-sealing layers of biaxially oriented polypropylene films, shrink packaging films, freeze-resistant films, etc. However, there are several problems with the production and quality of these copolymers. With respect to production, worthless noncrystalline polymers soluble in a polymerization medium are produced as a by-product in larger amounts than in the production of polypropylene, which results in a loss of monomers. These by-products are an economic disadvantage and create troubles in producing the copolymer. For example, an increase in stirring force due to an increase in the slurry viscosity is required, a reduction in heat transmission of a polymerization reactor results, and the like. Such phenomena become more serious when the content of the comonomer (e.g., ethylene, butene-1, etc.) in the copolymer increases to produce a copolymer having more excellent freeze resistance and heat-sealing properties.
On the other hand, with respect to their physical properties, the copolymers have a generally poorer blocking slip of film as compared with polypropylene, and are liable to lose their transparency with time. Such defects partly result from the poorer stiffness thereof as compared with polypropylene but are mainly the result of the high contents of low molecular weight non-crystalline polymers in the copolymer.
The above-described problems in production and quality are generally so related that an improvement in one results in a deterioration of the other. For example, as disclosed in Japanese Patent Publication No. 4992/69, a process comprising copolymerizing after forming a small amount of polypropylene using a titanium trichloride catalyst results in an increase in the content of the low molecular weight non-crystalline polymers contained in the copolymer, which does not improve quality. On the other hand, the amounts of worthless non-crystalline polymers soluble in a polymerization medium in the copolymer decrease. The use of a polymerization medium with more solvent power to reduce the content of low molecular weight non-crystalline polymers in the copolymer results in an increase of the amount of non-crystalline polymers dissolved in the polymerization medium. In some cases, this reduces the polymerization temperature which in turn reduces the polymerization activity of the catalyst. In this case to achieve the same polymerization amount in a given amount of time results in an increased amount of catalyst residue remaining in the copolymer, which leads to deterioration of heat stability and hue. Prolonged polymerization leads to a serious reduction in productivity.
On the other hand, turning to the catalyst, those catalysts which have generally been used for producing polypropylene comprise titanium trichloride and an organoaluminum compound such as diethylaluminum chloride. In these catalysts, the molar ratio of titanium trichloride to the organoaluminum compound has been 1:1 to 20 (see Polypropylene Resin, p. 26 (published by Nikkan Kogyo Shinbun Sha)). This is because catalytic activity and stereoregularity (indicated in terms of n-heptane insolubles or crystallinity) sharply decrease as the Al/Ti molar ratio in the catalyst components becomes less than 1, whereas catalytic activity and stereoregularity decrease when the Al/Ti molar ratio exceeds about 10. This is reported in various publications such as L. Reich and A. Schindler, Polymerization by Organometallic Compounds, Chapter III D (published by Interscience Publishers), T. Keii, Kinetics of Ziegler-Natta Polymerization, Chapter 4.2 (published by Kodansha), and the citations given therein and Japanese Patent Application (OPI) No. 34478/72 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application").
In the production of propylene copolymers, too, it is disclosed, for example, in Japanese Patent Application (OPI) No. 35487/74, that the molar ratio of an organoaluminum compound (B) to a titanium compound (A) [(B)/(A)] is about 0.5 to 20, preferably 1 to 10. Thus, it has been common knowledge to use catalysts with a (B)/(A) molar ratio within the above-described range.
However, the above-described processes still do not solve the aforesaid problems in the production of propylene copolymers containing about 80 to 99 mol% propylene and 1 to 20 mol% ethylene and/or an .alpha.-olefin having 4 to 18 carbon atoms, which are the end products of the present invention.